1. Field of the Invention
The present invention relates to an internal power supply voltage generating circuit and to reducing overshoot caused when a feedback control responds to a drop in the internal power supply voltage.
2. Description of the Related Art
High-capacity semiconductor memory devices such as DRAMs having storage capacity greater than 64 Mbits commonly use a low supply voltage (typically 3.3 V or less) to reduce power consumption. However, other DRAMs, integrated circuits and systems use a higher supply voltage (typically about 5 V). Accordingly, a flexible semiconductor memory device should be able to utilize either power supply.
Conventionally, a semiconductor memory device includes an internal power supply voltage generating circuit that generates a stable internal supply voltage from the external supply voltage. When a word line is enabled, the semiconductor memory device consumes a relatively large amount of power, which can cause the voltage level of the internal power supply voltage to drop. This is commonly called as a DIP. If a DIP causes the internal supply voltage to remain low, the low supply voltage can create a fatal error in the circuit.
To improve response to a DIP, an internal power supply voltage generating circuit can use a feedback system. When a DIP occurs, the feedback system detects a voltage drop and increases the driving ability of the internal power supply to rapidly recover a normal voltage level. However, the conventional method, which increases the driving ability to improve response to a DIP, can cause an overshoot where the internal voltage rises above the desired level. The overshoot can be serious enough to cause a fatal error in the internal circuit. Specially, with a low external supply voltage of 3.3 V, the overshoot is typically small and does cause serious errors, but an overshoot with a high external supply voltage of 5 V can causes a fatal error.
One way to decrease the problems with overshoot is to improve the current capacity of the internal power supply to reduce DIP and the need for a feedback response. Increasing current capacity typically requires enlarging the capacitance of the power supply circuit to improve response to a DIP. Accordingly, the capacitance for the power supply circuit occupies a large area in a chip and increases the total IC area and cost. Another way to decrease problems with overshoot is to reduce or eliminate occurrences of DIP. This method increases recovery time and reduces the current charging word lines, but this method changes the operating speed of the chip.
Another concern in the internal supply voltage generating circuit is that the circuits that solve the problem of overshoot for a high external supply voltage of 5 V can have detrimental consequences when used with a low external supply voltage of 3.3 V.